let's find out much compression of opcodes (not including their immediate operands) would reduce the code size of Super Mario Bros.
https://gist.github.com/notwa/1aba32c1dd5fa60862aa63e7b7665b6f
the answer is not quite 30%. i used Golomb coding here, but Huffman yields a very similar result.
also this code is kinda bleh (i even just spotted a var called 'asdf' and an off-by-one) but i don't feel like working on it anymore tonight.
would you have paid the price of dedicated decoding hardware to fit 30% more code in your NES game? i wonder…
if my interest in this persists, i might try this with megaman 1. it's a bigger cart to begin with, so it might take a lot more work to trace all of its subroutines, or maybe i'd just source the disassembly going around.
also neither of these games use more than 128 unique instructions, so you could just drop a bit and get 12.5% for free*.
* not including the LUT cost, but this is all theoretical anyway.
quick and *very* non-exhaustive test shows MM only(?) has ~25% more code and compresses a couple percent better.